I've heard this too, I believe it's anything 10 years or older. I've also heard that if any airbags deploy they instantly write them off regardless of age as they are quite costly to replace. My sister's 19 reg Golf R (such a boring car to drive

) got rear ended a few months back, on the outside it didn't look too bad, rear bumper, a slight crease in the nearside rear 1/4 panel etc but the main damage was to the rear suspension as it had moved forward on impact, anyway the insurance sent it to VW for the repairs & along the way the insurance were keeping my sister updated with parts & what the parts were costing them etc, the rear wheel had to be replaced which was an astonishing £1,100

just for one rim without a tyre as being the R version it has diamond cut rims. The parts prices along with labour from VW came very close to writing the car off & the insurance stated to my sister that when they were adding up all the cost of repairing the car & they got to their grand total one more thing would have declared it a write off.